Because organic electroluminescent (EL) display devices using a self light emitting organic EL element do not require a backlight as do liquid crystal display devices, EL display devices are advantageous for reducing the thickness of displays. For that reason, and because the viewing angle of EL display devices is not restricted, it is widely anticipated that development of EL display devices will lead to their becoming the next generation of display devices. The organic EL element used in an organic EL display device also differs from a liquid crystal cell in that, while the display in each liquid crystal cell is controlled by an applied voltage, in an organic EL element, the luminance of each of light emitting element is controlled by the value of the current flowing through the element.
As an active matrix system is, compared to a passive matrix system, advantageous in terms of both extending the life of an organic EL element and enlarging screen size, much research and development has focused on development of active matrix systems. Among the proposals that have resulted are a current value modulation method in which gradation display is performed by maintaining a constant light emission period while fluctuating the magnitude of the luminance during each frame of that period, and a time division method in which the gradation display is performed by maintaining a constant emission luminance during the light emitting period while fluctuating the light emission period of the organic EL element.
FIG. 9 shows a pixel circuit in an organic EL display device of the active matrix system of the time division method of a conventional art, such as that disclosed in Japanese Patent Application No. 2002-149113 (Page 24, FIG. 8; Page 25, FIG. 9). From a scanning line driving circuit 106, two scanning lines consisted of a first scanning line 101 and a second scanning line 102 extend to respective pixels. A source supply circuit 107 supplies a positive voltage VDD and a negative voltage VSS to the respective pixels. A signal line driving circuit 108 supplies signal voltages to the respective pixels through a signal line 103. The first scanning line 101 is connected with the gate of a switching element 109, and the n-channel switching element 109 turns on or off a connection of the signal line 103 with the gate of a p-channel driver element 104. The second scanning line 102 is connected with the gate of a discharge switch element 110, and the discharge switch element 110 turns on or off a connection of the positive source VDD with the gate of the driver element 104. To the discharge switch element 110, a capacitance 111 is connected in parallel. One end of the driver element 104 is connected with the positive source VDD and the other end thereof is connected with the negative source VSS via alight emitting element 105.
Accordingly, when the discharge switch element 110 is turned on, both ends of the capacitance 111 are short-circuited for discharging. When the discharge switch element 110 is turned off and the switching element 109 is turned on, a signal voltage of the signal line 103 is written in the capacitance 111, and, in accordance with the written voltage, the driver element 104 and, therefore, the light emitting element 105 are turned on or off.
According to the conventional art, the light emitting period in one frame is determined by a combination of emission off or on subframes which is assigned a weight of 0 to n bits, thus display gradations in accordance with luminance data obtained therefrom. In the conventional art, a problem of visibility called false contours is reduced for a bit having a comparatively longer light emitting period by arranging the bits by dividing and dispersing the light emitting period on the time axis.
In the method, the driver element 104 has a function as a switch for turning on or off the current flowing to the light emitting element 105. In other words, to a gate voltage of the driver element 104 is applied either an on-voltage sufficiently larger than a threshold voltage of the driver element 104 or an off-voltage sufficiently smaller than the threshold voltage. Because the impedance of the driver element 104 is sufficient smaller than the impedance of the light emitting element 105 when the driver element 104 is turned on, the value of the current flowing to the light emitting element 105 while the light emitting element 105 is emitting light is determined by the impedance of the light emitting element 105. Therefore, the influence of inter-element variations of the threshold voltage, mobility, and the like of the driver element 104 are reduced. Accordingly, if the light emitting element 105 maintains uniformity within the display device, the display device is capable of displaying a high quality image having satisfactory uniformity with reduced false contour.
FIG. 11 shows a pixel circuit in an organic EL display device of the active matrix system of the time division method of another conventional art (See, for example, Kageyama et al, “51.1: A2.5 inch OLED Display with a Three-TFT Pixel Circuit for Clamped Inverter Driving”, SID04DIGEST, p 1395, FIGS. 3 and 4.). From a scanning line driving circuit 206, two scanning lines consisted of a first scanning line 201 and a second scanning line 202 are extended to respective pixels. A source supply circuit 207 supplies a positive source VDD (positive source voltage VDD) and a negative source VSS (negative source voltage VSS) to the respective pixels. A signal line driving circuit 208 supplies signal voltages to the respective pixels via a signal line 203. The signal line 203 is connected with the gate of a driver element 204 via capacitance 211, and the source of the driver element 204 is connected with the positive source VDD.
The first scanning line 201 is connected with the gate of an n-channel first switching element 209, and the first switching element 209 turns on or off the connection between the gate and the drain of the p-channel driver element 204. The second scanning line 202 is connected with the gate of an n-channel second switching element 210, and the second switching element 210 is provided between the drain of the driver element 204 and an anode of the light emitting element 205 for turning on or off the connection between the second switching element 210 and the driver element 204. Accordingly, in a state where the second switching element 210 is turned on, the current flowing to the driver element 204 flows to the light emitting element 205.
In such a circuit, as shown in FIG. 12, in a state where a signal voltage is supplied to the signal line 203, the first switching element 209 and the second switching element 210 are turned on and then only the second switching element 210 is turned off. By this operation, current from the positive source VDD flows to a gate electrode of the driver element 204 until the voltage between the source and the gate of the driver element 204 comes up to be the threshold voltage of the driver element 204, in a state where the gate and the drain of the driver element 204 are short-circuited, and the difference at this time between the threshold voltage of the driver element and the signal voltage is set at the gate of capacitance 211. Then, the first scanning line 201 is set at L level for turning off the first switching element 209, thus a charging voltage of the capacitance 211 is determined.
Such writing operation of the signal voltage is performed in parallel for pixels of respective columns within one row, and the operation is sequentially performed for respective rows (n rows in FIG. 12). In one frame period, a writing period of the signal voltage in all pixels is first executed, and after writing is complete all pixels enter into a light emitting period.
During the light emitting period, a triangular wave is applied as a reference voltage to the capacitance 211 through the signal line 203, and, during a period where the voltage of the triangular wave is lower than the signal voltage written in the pixels during the data writing period, the driver element 204 is turned on to cause the light emitting element 205 to emit light. According to this method, because the threshold voltage of the driver element 204 can be compensated for, influence of variations of the threshold voltage of the driver element 204 can be further reduced than by the method shown in FIG. 9.
According to this method, decentralized processing for a bit having long light emitting period is not required, as shown in Patent Document 1, and the light emitting element always emit light with an apex of the triangular wave as the center of gravity, thus not causing to generate a visionary problem of principally false contour, and therefore a high quality image with satisfactory uniformity can be displayed.